This invention relates to solid propellant compositions and to a method for their cure. More particularly, this invention concerns itself with an improved catalyst system for curing polyurethane propellant compositions.
In general, solid propellant compositions are composed of a mixture of an inorganic oxidizer disposed in a matrix of plastic resinous or elastomeric material. The matrix provides fuel for the combustion reaction and, in some propellants, acts as a binder for holding the propellant mixture together prior to combustion. Amongst the preferred binder materials are the polyurethanes which have proven to be an excellent fuel and binder component for modern propellants.
The Urethane based propellants are conventionally cured at temperatures of from about 110.degree.-1135.degree. F.. The use of these high temperatures, however, often creates problems of stress and strain in case bonded solid rocket motors. A reduction of the cure temperature to 70.degree.-80.degree. F. would significantly reduce propellant bore strains and bond stresses in case bonded solid rocket motors. The lower cure temperature could also result in better propellant mechanical behavior, because side reactions are also minimized at lower temperatures.
As a consequence of the problems created by high temperature curing, a considerable research effort has evolved in an attempt to develop catalysts which promote the isocyanate-hydroxy reaction at room temperature under the conditions expected in a solid propellant environment. Also, the catalysts have to achieve their effectiveness without seriously affecting propellant processing characteristics, mechanical behavior and storage stability.
One catalyst which has been suggested for use in curing polyurethane propellant is Ferric acetylacetonate, otherwise designated as Fe(AA).sub.3. However, a satisfactory balance between potlife and the time required for full cure is a major problem with such catalysts since the accelerating effect of a higher curing temperature on the urethane reaction cannot be utilized. This is particularly true for a lithium initiated, hydroxy terminated polybutadiene prepolymer cured with a diisocyanate, such as hexamethylene diisocyanate, since both NCO groups of which possess equal reactivity.
At catalyst levels as low as 0.001% Fe(AA).sub.3 the propellant mix will be castable for only 30 minutes in case of hexamethylene diisocyanate and somewhat longer for toluene diisocyanate cured propellants but still requires 7-10 days for full cure. In the absence of catalyst Li HTPB propellants are practically uncurable (weeks at 180.degree. F. are required). Reduction of the catalyst level below 0.001% entails the danger of losing the catalyst in degradative reactions.
With the present invention, however, it has been found that Fe(AA).sub.3 can be effectively utilized as a catalyst for the low temperature cure of polyurethane based propellants, provided it is employed in a system along with a suppressor material and a scavenger material. Such a catalyst system has proven to be highly effective in solving the problems encountered in the curing of polyurethane binders and propellants. The suppressor material can be either a diketone or an acid while zinc oxide has been found to be a most efficient scavenger component.